1. Filed of the Invention
This invention relates to a method for preparing a polyorganosiloxane that contains mono- and tetrafunctional units, by hydrolysis and condensation of a mixture of alkyl silicates and alkylsilanes and/or their hydrolyzates and compounds containing SiCl groups.
2. Discussion of the Related Art
Methods for preparing polyorganosiloxanes that consist of monofunctional M-units (R.sub.3 SiO.sub.1/2) and tetrafunctional Q-units (SiO.sub.4/2) are known. Most of the methods use tetraalkyl silicates as the source of Q-units. The M-units are usually introduced in the form of hydrolyzable trialkylsilanes. The hydrolysis/condensation reaction taking place must be catalyzed in order to obtain the desired products. Acidic compounds have proved to be especially suitable for that purpose. The reaction is usually carried out in an organic solvent. A method described in U.S. Pat. No. 2,857,356 starts with a mixture of tetraalkyl silicate and a trialkylchlorosilane in a solvent immiscible with water, and the mixture is reacted with water, with the resin solution being separated from the alcoholic hydrochloric acid by phase separation. The reaction is difficult to control because of the two-phase system. Additional drawbacks include the production of large amounts of concentrated alcoholic hydrochloric acid and the shift of the stoichiometric balance by the formation of volatile byproducts, and the siloxanes containing Q-units dissolved in the aqueous hydrochloric acid. The resins contain a large fraction of OH groups bonded to silicon.
To minimize the production of alcoholic hydrochloric acid, the use of a trialkylalkoxysilane and/or of its disiloxane and aqueous hydrochloric acid instead of trialkylchlorosilanes is proposed in EP 195 936. In this case, the alkyl silicate is added to the heterogeneous mixture of silanes and hydrochloric acid. Usable products are obtained by this method when the mixture contains at least 5 wt. % HCl. For this reason, hydrochloric acid with at least 10 wt. % hydrogen chloride has to be used. Volatile oligomers are also formed, which adversely affect the stoichiometric balance. Since the system is heterogeneous throughout the entire time of reaction, the reaction is difficult to control.
For this reason, it has been suggested to react triorganoalkoxysilanes and/or their hydrolyzates with tetraalkyl silicates in the presence of strongly active catalytic compounds, which can then be used in smaller amounts. Thus, EP 529 547 describes the use of alkyl silicate and disiloxane in combination with a strong proton acid, such as a sulfuric or phosphoric acid, with concentrated hydrochloric acid as catalyst. EP 294 277 uses a sulfonic acid and phosphonitrile chloride for the reaction of alkyl silicates with oligomeric siloxanes. In EP 604 847, a multi-proton acid, preferably sulfuric or phosphoric acid, is used, which is neutralized in several stages. Again, EP 640 109 describes the use, in particular, of hydrochloric acid, but only in catalytic amounts (preferably 0.2 to 50 mmoles based on 1000 g of reaction mixture), which usually makes it possible to carry out the procedure in a homogeneous phase. The infeed (addition) of small amounts of hydrochloric acid, and especially its homogeneous distribution in the reaction mixture, seems to be a problem (in Example 1, 1.9 g of 20% hydrochloric acid is used in relation to 1475 g of reaction mixture), since, according to EP 640 109, homogeneity is not achieved when the acid concentration is raised.
The critical drawback to such methods, in which the catalytically active compounds are fed into the silane and/or siloxane mixture, consists of the fact that partially high catalyst concentrations occur at the point of infeed, either because of the insolubility of water or aqueous acids in the siloxanes used, or because of the usual infeed, which leads to uncontrollable reactions taking place at this point. The catalyst can be distributed through the entire reaction mixture only after the formation of silanols with solubilizing action, and the catalyst is present in the specified (average) concentration only thereafter.